The present invention relates generally to collimation techniques and, more particularly, to an apparatus and method for controllably positioning multiple optical fibers relative to respective collimating lenses.
It is desirable in many applications to collimate the optical signals emitted by each of a plurality of optical fibers. For example, the optical signals emitted by a plurality of optical fibers are preferably collimated prior to transmission to achieve the smallest divergence optical beam and to direct the greatest portion of the optical energy from all fibers onto the smallest target point in the far field. By collimating optical signals prior to transmission and by steering each collimated beam in the same direction, the greatest power arrives at the target point in the far field.
Optical signals emanating from a small source region are generally collimated by placing the source region at the focal point of a lens. Optical signals from an optical fiber emanate from a small region and are generally collimated by means of a collimating lens disposed proximate one end of an optical fiber. Optical signals are typically coupled into an optical fiber by focusing onto the end of an optical fiber by means of a focusing lens. The collimating and focusing operations are complementary, interchangeable, and reversible. The same lens, i.e., a collimating lens, both collimates the diverging optical signals emitted by an optical fiber and focuses collimated light onto the optical fiber. In order to minimize transmission losses due to the lens, the lens is preferably coaxially positioned with respect to the optical fiber such that the end of the optical fiber is coincident with the focal point of the lens. If the fiber is moved along the collimating lens axis and positioned such that the end of the optical fiber is offset from the focal point of the collimating lens, a transmitted beam will be either convergent or divergent, thereby decreasing the concentration of energy that reaches a target spot in the far field. If the end of the fiber remains at the focal distance from the lens but is displaced from the lens axis, the beam will steer away from the target point. Similarly, during the reception of optical signals, if the lens is moved off axis or is positioned such that the end of the optical fiber is offset from the focal point of the lens, the focal point of the received signals misses the end of the optical fiber. Moreover, even if the lens is properly positioned, the lens will be unable to precisely focus the optical signals onto the end of an optical fiber if the received signals are not collimated and are, instead, divergent or convergent. Thus, efficient signal transmission demands that the optical signals be collimated.
Several techniques have been developed to determine if optical signals have been appropriately collimated. One technique for determining the collimation of highly coherent optical signals utilizes a Shearing interferometer having a slightly wedged glass plate which creates an interference pattern by overlapping reflections from the front and rear surfaces of the slightly wedged plate. The resulting interference pattern provides information relating to the degree of collimation of the original wavefront. By monitoring the interference fringes generated by the Shearing interferometer as a collimating lens and an end of an optical fiber are moved relative to one another, the degree of collimation provided by the collimating lens can be optimized. In instances in which the optical signals are incoherent, a Shearing interferometer is useless, and the collimation of the optical signals is typically performed in a less exact manner by moving the collimating lens and the end of the optical fiber relative to one another and subjectively determining when the resulting spot xe2x80x9clooks goodxe2x80x9d in the far field.
Conversely, in order to properly position a focusing lens relative to the end of an optical fiber, a test source typically provides collimated optical signals that are focused by the lens onto an optical fiber and the resulting optical signals that emerge from the other end of the optical fiber are measured with a power meter. In order to insure that the optical signals provided by a coherent test source are collimated, a Shearing interferometer can be utilized. By moving the focusing lens and the end of the optical fiber relative to one another and by measuring the respective power levels of the optical signals emitted by the optical fiber, the focusing lens can be positioned in that location which results in the largest percentage of the optical signals being focused onto the end of the optical fiber as evidenced by the maximum power reading. If the test source is collimated, then maximum power is read at the true focus.
By way of example, one technique for aligning an optical fiber within a connector has been described by U.S. Pat. No. 4,509,827 which issued Apr. 9, 1985 to Steven J. Cowen (hereinafter the Cowen ""827 patent). The alignment system described by the Cowen ""827 patent includes a light emitting diode for emitting optical signals and a directional coupler for coupling the optical signals into an optical fiber. The alignment system of the Cowen ""827 patent also includes a graded index (GRIN) lens disposed within a connector body for receiving signals from the optical fiber and a reference mirror placed orthogonal to the preferred light direction for reflecting the optical signals back through the GRIN lens and into the optical fiber. The alignment system of the Cowen ""827 patent further includes a photodetector, also connected to the directional coupler, for receiving the reflected optical signals. In order to appropriately align the optical fiber within the connector body, the alignment system includes a micropositioner that moves the end of the optical fiber relative to the GRIN lens until the photodetector indicates the maximum power level. Thereafter, the optical fiber can be fixed in position relative to the lens within the connector body, such as by potting the end of the optical fiber with a suitable material. The alignment technique described by the Cowen ""827 patent is designed to align a single optical fiber to a single lens. In applications in which a plurality of optical fibers must be aligned, the alignment technique described by the Cowen ""827 patent results in each fiber being suitably aligned to a corresponding lens, but the resulting, many collimated beams will point in as many directions.
A fiber optic collimation apparatus and an associated method are therefore provided that utilize a common reference mirror to collimate the optical signals emitted by a plurality of optical fibers. In this regard, the fiber optic collimation apparatus and method of the present invention can optimally position each of a plurality of optical fibers relative to respective collimating lenses.
The fiber optic collimation apparatus includes a plurality of optical fibers for providing respective primary optical signals and a plurality of collimating lenses, each of which is positioned to collimate the primary optical signals provided by a respective optical fiber. In one embodiment, the plurality of collimating lenses lies in a common lens plane such that each collimated optical signal is parallel to all other collimated optical signals after suitable adjustments have been made according to this method. The fiber optic collimation apparatus also includes a common reflector spaced from the plurality of collimating lenses and having a surface that defines a reflection plane orthogonal to the preferred direction of propagation of the collimated optical signals. The common reflector therefore reflects each of the collimated optical signals to produce respective return optical signals that are directed to the respective collimating lenses and, in turn, to the respective optical fibers. The fiber optic collimation apparatus further includes at least one detector for receiving the return optical signals and for determining a magnitude of the return optical signals. In one embodiment, for example, the fiber optic collimation apparatus includes a plurality of detectors for receiving return optical signals from respective optical fibers.
The fiber optic collimation apparatus also includes a plurality of actuators connected to respective optical fibers for iteratively positioning each optical fiber in a plurality of positions relative to the respective collimating lens. For example, each actuator can advantageously be a microelectromechanical optical fiber alignment device for positioning a respective optical fiber, typically in each of three orthogonal axes. The detector is designed to determine the magnitude of the return optical signals for each of the plurality of positions in which the actuator places the respective optical fiber. The actuators can thereafter fix the position of the optical fibers based upon the magnitude of the return optical signals. In particular, the fiber optic collimation apparatus can include a controller for controlling the actuators based upon the magnitude of the return optical signals received by the detectors. Typically, the controller directs the actuators to fix the position of each optical fiber at a position in which the return optical signals for the respective optical fiber have the maximum magnitude. By maximizing the magnitude of the return optical signals, the ends of the optical fibers will be positioned at the focal points of the respective collimating lenses such that the primary optical signals emitted by the optical fibers can be properly collimated and directed by the lenses and the return optical signals can be appropriately focused onto the ends of the respective optical fibers.
In order to position each optical fiber iteratively in a plurality of positions relative to the respective collimating lens, an actuator may first defocus the collimating lens by moving the respective optical fiber such that the detector indicates a return optical signal having a small magnitude. Thereafter, the actuator can iteratively position the respective optical fibers such that the detector indicates return optical signals having greater magnitudes. The controller can then determine the maximum magnitude of the return optical signals and the actuator can thereafter position the respective optical fiber with respect to the respective lens so as to produce return optical signals having the maximum magnitude, thereby evidencing that the end of the optical fiber is at the focal point and on the optic axis of the lens.
A fiber optic collimation apparatus can also include a plurality of light sources for providing the primary optical signals for purposes of alignment. In addition, the fiber optic collimation apparatus can include a plurality of directional couplers. Each directional coupler interconnects the respective light source, optical fiber and detector. Thus, each directional coupler provides the primary optical signals from the respective light source to the optical fiber and receives the return optical signals from the optical fiber for routing to the respective detector.
In operation, a plurality of primary optical signals is provided by respective optical fibers. Each of the plurality of primary optical signals is then individually collimated to produce respective collimated optical signals. Each collimated optical signal is reflected from a common reflection plane disposed orthogonal to the preferred direction of propagation of the collimated optical signals, thereby creating a return optical signal from each collimated optical signal. The magnitude of each return optical signal is then determined. The optical fiber is iteratively positioned in a plurality of different positions with the magnitude of the return optical signals being separately determined at each position. Thus, the position of each optical fiber can be fixed in a final position based on the magnitude of the return signals. In particular, the position of each optical fiber can be fixed such that the return optical signals have a maximum magnitude, thereby effectively positioning the end of the optical fiber at the focal point and on the optic axis of the respective lens.
Note that if the lenses in the array in the lens plane are placed with slight errors in angle or position so that all lens axes are not exactly parallel or exactly spaced, the alignment process will still result in co-collimated beams, all pointing in the same direction orthogonal to the reference reflector, however the associated fiber ends may not lie exactly on the axes of the respective lenses and not be regularly spaced, thus compensating for the errors.
The fiber optic collimation apparatus and method of the present invention therefore permits a plurality of optical fibers to be precisely positioned relative to respective collimating lenses such that the optical signals emitted by the optical fibers are collimated and such that collimated signals that are received by the collimating lenses are properly focused upon the end of the respective optical fibers. Thus, the fiber optic collimation apparatus and method of the present invention should reduce transmission losses, both in signal power and signal integrity. Notably, the fiber optic collimation apparatus and method efficiently position a plurality of optical fibers relative to respective collimating lenses by reflecting the collimated optical signals from a common reflector, thereby assuring that all of the plurality of collimated beams point in the same preferred direction.